This invention relates to an improvement to measuring instruments for measuring the relative position of two objects shiftable with respect to one another in a first guide, of the type comprising a scale which defines a measuring graduation extending along a measuring direction, at least one reference mark absolutely positioned with respect to the graduation, a scanning unit adapted to scan both the graduation and the reference mark and to generate scanning signals in response thereto, and an evaluating unit coupled to receive the scanning signals.
In such a measuring insrument the electrical control impulses generated by means of the reference marks can be used in various ways. For example, such control impulses can be used to set a reproducible zero position in the evaluating unit, to load a predetermined position value at the beginning of the measurement operation, or to monitor the measured position value against interference impulses, as well as to act on a control arrangement engaged on the outlet side of the evaluating unit.
In West German patent DE-PS No. 29 52 106 there is described an incremental length or angle measuring instrument in which a scale is provided which defines a graduation and an array of reference marks alongside the graduation. Each of the reference marks is provided with a different line-group distribution. The individual reference marks are scanned by scanning fields on a scanning plate in a scanning unit, each scanning field being clearly allocated to a respective one of the reference marks, because the allocated scanning field has the same line-group distribution as the respective reference mark. In order to provide clear, unambiguous scanning of these reference marks, the spacing between the scale and the scanning plate is preferably at most no more than about 2d.sup.2 /.lambda. because of the irregular line-group distributions. In the foregoing expression the symbol d has been used to signify the width of the narrowest line of the line division of the reference marks and .lambda. has been used to signify the center-of-gravity wavelength of the illuminating light.
It is also known that in order to obtain optimum scanning of a regular periodic incremental graduation of a scale, the separation between the scale and the scanning plate is not limited to only a single definite spacing. Rather, a number of different spacings are possible. If the graduation of a scale is illuminated by collimated light having a parallel beam course, then there are certain planes behind the graduation plane of the scale which can be scanned with scanning graduations of like grid constant in an optimal manner. This is due to interference effects of the light rays diffracted by the graduations of the scale, which produce diffraction images of the graduation of the scale. With a grid constant P.sub.M of the graduation of the scale and a center-of-gravity wavelength .lambda. of the light, these planes have spacings n.multidot.p.sub.M.sup.2 /.lambda. (n=0, 1, 2 . . . ) from the graduation plane of the scale. Optimal electric scanning signals can therefore be generated at spacings between the graduation of the scanning plate and the graduation plane of the scale equal to n.multidot.p.sub.M.sup.2 /.lambda. (Machine Shop Magazine, April 1962, page 208). The use of greater spacings between the scale and the scanning plate brings important advantages in that spacing tolerances are often greater with greater spacings, in which case lower demands have to be made on the precision guiding of the scanning plate with respect to the scale. Furthermore, with greater spacings the periodic scanning signals generated in the scanning of the incremental graduation of the scale have a more sinusoidal wave form, so that the signal period of the scanning signals can better be subdivided by interpolation techniques. The utilization of very large spacings between the scale and the scanning plate in the scanning of the incremental graduation is possible without interposed focussing optics only in measuring instruments in which the scale of the instrument includes no reference marks. This is because the clear scanning of the reference marks as set out above requires a predetermined small spacing between the scale and the scanning plate which must not be exceeded.